Voicemail Delivery is VERY Delayed

My dad has an iPhone XS max and recently it started not delivering voicemails when they are left. He randomly had voicemails come in last week from over a month ago. Similar things have been happening for a few weeks. As this is a business line for him that is a major problem.

He ran updates and troubleshooting with Apple and they concluded it was a carrier issue. He went in to the carrier and they said it was a phone issue.

Anyone else have this problem and how did you fix it? Any recommendations? Thanks